A number of bonding techniques are known for joining semiconductor substrates including intermediate-layer bonding (such as adhesive, glass frit, or solder), anodic bonding, thermal compression bonding, fusion bonding, etc. The selection of a bonding technique to be used for specific substrates may vary depending on thermal budget, hermeticity requirements, properties of the bond interface, and so on.
Fusion bonding can join two substrates without an intermediate layer; however, fusion bonding includes using high annealing temperatures that approach 900 degrees Celsius to achieve good bond strength. Due to the high temperature requirement, fusion bonding is limited to a few niche applications such as silicon-on-insulator (SOI) substrate fabrication, limited MEMS assembly, etc. The high temperatures used in fusion bonding is not compatible with complementary metal oxide semiconductor (CMOS) devices.
In contrast to fusion bonding, plasma-activation of semiconductor bonding surfaces enables direct covalent bonding at significant lower temperatures and has found applications from silicon-on-insulator (SOI) substrate fabrication to 3-dimensional substrate stacking. The traditional plasma surface activation process involves polished bond interfaces receiving a brief O2 or N2 plasma treatment followed by a wet-dip process in a standard clean one (SC1) or de-ionized water bath. The post-plasma wet dip process step increases the bond strength of the plasma enhanced bonding. Unfortunately, the post-plasma wet-dip process is not compatible with most MEMS devices or substrates that include desiccants which cannot generally be exposed to water. More specifically, the traditional post-plasma wet-dip process increases the likelihood of problems in the MEMS due to stiction.
One alternative to the traditional post-plasma wet-dip process is to hydrate the plasma treated interface by exposing it to ambient for an extended period of time. However, this approach in general is difficult to control and the bond interface can react with contaminants in air which degrade the bonding surface energy.